Plastic containers and paper containers have so far been used widely in packaging foods and drugs as well. Furthermore, recently, attempts have been made to make use of such characteristics features thereof as easy moldability and light weights and widen the field of use thereof to those containers for liquid food products and the like the mainstream of which has so far been represented by metal cans and glass bottles.
Plastics and paper, when considered as base materials for packaging materials, have many excellent characteristic features in addition to those mentioned above. As compared with metals and glass, however, they are basically inferior in the ability to block the passage of gases such as oxygen and water vapor (gas-barrier properties), readily allowing the contents to become deteriorated or putrefied as a result of oxidation or moisture absorption. When they are used as packaging containers for carbonated drinks, for example, they present a drawback that carbon dioxide escapes, leading to the loss of the cooling and refreshing sensation, and the like.
Therefore, in cases where a plastic or paper is used as the base material for packaging containers, the packaging containers are provided with a gas-barrier layer made of a specific material in addition to the base material layer so that the gas-barrier properties thereof may be improved.
In cases where a plastic container is to be provided with a gas-barrier layer, the simplest method employed in the art comprises coating the container with a vinylidene chloride resin, which is excellent in gas-barrier property. However, the vinylidene chloride resin, which is a chlorine-containing compound, is currently a material to be kept at a distance since it may cause the formation of hazardous substances on the occasion of disposal and incineration.
Furthermore, the method comprising providing a gas-barrier layer using an aluminum foil or aluminum vapor deposited film, or a metal oxide vapor deposited film is now also employed and it is very effective for the purpose of obtaining packaging containers with high gas-barrier properties. However, these materials have problems, for example; they are expensive and increase the cost of containers, and the use thereof is restricted to those containers which have a form allowing the lamination with such a film-shaped material. Furthermore, the aluminum foil and aluminum vapor deposited film cannot be used in those containers required to be transparent. Another problem is, for example, that they damage incinerators on the occasion of incineration. Metal oxide vapor deposited films also have further problems; for example, they readily allow peeling off or cracking of the vapor deposited layer and are difficult to handle.
Furthermore, recently, researches have been made concerning the method of providing a high gas-barrier layer at low cost by coating with an eco-friendly chlorine-free compound resin, for example a polyvinyl alcohol resin, ethylene-vinyl alcohol copolymer (EVOH) or the like. These resins are characterized by their high crystallinity, and polyvinyl alcohol resin species higher in crystallinity show very good gas-barrier properties in a dry condition but they are highly hygroscopic and therefore have a problem in that, under high humidity conditions, their crystallinity falls and their gas-barrier properties deteriorate. In the case of the ethylene-vinyl alcohol copolymer, their gas-barrier properties are deteriorated under high humidity conditions to a lesser extent as compared with the polyvinyl alcohol resin but their gas-barrier property level is generally unsatisfactory.
Therefore, coating material compositions with gas-barrier properties in which a resin having a gas-barrier property and an inorganic layered compound are used in combination have been investigated to solve the problems discussed above. Thus, Japanese Kokai Publication Hei-05-140344 discloses a method for forming a gas-barrier layer composed of a specified amount of an ethylene-vinyl alcohol copolymer resin and an inorganic filler. However, the packaging containers obtained by this method still have problems; for example, their gas-barrier properties are still at low levels under high humidity conditions, and high levels of gas-barrier properties are obtained only when the gas-barrier layer is sufficiently thick. When, using such coating material, the coating layer is thickened for the manifestation of sufficient gas-barrier properties, the weight increases, making it difficult to transport and handle the containers and, further, increasing the cost thereof. Accordingly, a coating material with a gas-barrier property capable of manifesting satisfactory gas-barrier properties even when the gas-barrier layer thickness is reduced as far as possible is earnestly demanded.
On the other hand, paper packaging containers mainly occur as box-shaped containers because of the difficulty in manufacturing complicatedly shaped ones. However, they can be shaped without using any special molds, heat or the like and are highly opaque to light and, nowadays, there is a growing demand for them as paper/plastic film composite materials, in particular as containers for drinks such as sake and milk, which are readily deteriorated upon exposure to ultraviolet rays. However, the material used as the gas-barrier layer is an aluminum foil or aluminum vapor deposited film, so that the special feature of paper containers that they can be incinerated cannot be utilized in this case. Thus, a substitute gas-barrier layer-forming material sufficiently reduced in loading potential in incineration has been demanded.